4-Nitro-phenylalanine Bromides: D−π–A Structural Features and Their Nonlinear and Linear Optical Properties

Nonlinear optical materials have important applications in optical communication, information processing, biomedicine, and other fields. Herein, two organic chiral 4-nitro-d(l)-phenylalanine bromides (4NDPAB and 4NLPAB) were successfully synthesized and fully characterized. Single-crystal X-ray diffraction data reveal the P2₁ non-centrosymmetric structure, in which the chiral cations and Br^– anions are linked through four intermolecular hydrogen bonds. Chirality is identified by circular dichroism spectroscopy. One enantiomer, 4NDPAB, is described as an example. It exhibits an absorption edge at 435 nm (E_g = 2.85 eV) and demonstrates a high thermal stability up to 230 °C. Additionally, 4NDPAB shows strong second harmonic generation (SHG), 1.8 times higher than that of commercial KDP, and large birefringence (Δn_exp = 0.178). Density functional theoretical calculations and analyses confirm that the strong SHG intensity and excellent anisotropy performance are due to the D−π–A structural nature of 4-nitro-phenylalanine and its parallel crystal packing arrangement.